1. Field of the Invention
The present invention relates to systems and processes used in manufacturing integrated device die, such as integrated circuits and laser diodes, including diode lasers formed on conductive substrates. More particularly, the present invention provides for securing wafers having conductive substrates, during the process of cutting the wafers into individual die, and further provides for securing the die separated from the wafers during and after the wafer cutting process.
2. Description of Related Art
Sapphire Al2O3 is used as a substrate for the growth of Gallium Nitride GaN in commercial laser diode manufacturing systems, and can also act as the substrate of the finished product. However, the use of sapphire substrates introduces certain problems.
For instance, sapphire is an electrical insulator and this causes problems when it is used as a wafer substrate in the fabrication of laser diodes. Because it is an insulator, electrical contacts to the diodes are usually placed on the wafer""s active surface, and these contacts occupy areas that would otherwise be utilized for generation and emission of light.
Efforts have been made to implement laser diodes using GaN with other substrates. These approaches typically involve removal of the GaN from the sapphire substrate on which it is grown, and then remounting it on another substrate. Advantages of this approach arise because copper or other metal substrates are excellent heat and electric conduction materials. A light emitting diode or laser diode LED with a metal substrate can be driven with higher current and yield brighter output. In addition, the device with good electric conduction to the substrate requires only one wire bonding on the active surface, and yields higher output. Furthermore, the sapphire substrate used for growth of the GaN may be reused for reduced cost.
For example, U.S. Pat. No. 6,365,429 teaches a method by which xe2x80x9cremoval of the sapphire substrate after growth of the laser diode array structures simplifies providing electrical contacts to the laser diode arrays and avoids special architectures while allowing a superior heat sink to be attached to the laser diode arrays. The laser diode array may be attached to a thermally conductive wafer before or after substrate removal by soldering, thermo-compression bonding or other means.xe2x80x9d (col. 2 II. 20-28)
However, no known method or tools to dice this type of wafer have been applied on a commercial scale.
Present methods of separating a wafer based on a sapphire or crystalline semiconductor substrate into die involve scribing the wafer after first adhering the wafer to a flexible sheet, known as xe2x80x9cblue tapexe2x80x9d. After scribing, mechanical pressure is applied to break the wafer along the scribe lines, leaving the die attached to the flexible sheet for their subsequent removal.
However, wafers having metal substrates cannot be separated into die using scribing techniques. Rather, wafers having a metallic substrate, for example one made of copper, must be cut completely through to obtain separated die. Cutting completely through the wafer would damage an adhesive sheet attached to the wafer, unless very precise control of the cutting process were possible. Furthermore, if an adhesive sheet is not attached to the wafer prior to cutting the die, in order to avoid the damage, the separated die would be difficult to handle during and after the cutting of the wafer. Thus, there is a need for a method and system for securing both the wafer and the separated die during and after the cutting of the wafer.
It is desirable, therefore, to provide a system and method for dicing wafers having conductive or metallic substrates, for use in fabricating die in large volume, in an efficient manner that maximizes the die-manufacturing yield. Furthermore, it is desirable that such a system be compact, safe to operate, and low cost.
Therefore, it is an object of the present invention to provide for securing a substrate to a mounting surface in order to perform the cutting substantially through the substrate, thereby permitting separation of the substrate into die in accordance with a cutting pattern. It is a further object of the present invention to provide for securing the separated die to the mounting surface, both during and after the cutting process.
One embodiment of the invention provides a method for dicing a wafer, comprising mounting the wafer on a porous member having a mounting surface; securing the wafer on the mounting surface by applying suction to the wafer through pores in the porous member; and dicing the wafer into individual die, the die remaining secured to the mounting surface by the applied suction.
The present invention provides a method comprising mounting a wafer having substrate, and carrying an array of integrated devices, on a stage such as a movable X-Y stage further comprising a vacuum chuck provided with a porous mounting surface. Applying suction through the pores of the mounting surface secures the wafer to the mounting surface. The wafer is cut in one embodiment by directing laser energy at a surface of the wafer using a solid-state laser, to form a plurality of kerfs substantially through the thickness of the wafer, thereby dicing the wafer. Cutting a wafer by the present method cuts kerfs through the wafer, the kerfs having a width preferably in the range of 10 to 20 microns.
The present invention is suitable for manufacturing blue laser diodes based on Gallium Nitride structures that have been removed from the substrate on which they were grown, and then mounted on a conductive substrate. According to the present invention, greater device density on the wafer, and greater manufacturing yield are achieved, while also reducing the time required for dicing the wafer into individual die. Furthermore, the present invention is based on compact, low-cost machines, and otherwise reduces the overall manufacturing costs for such integrated device die.
In accordance with the present invention, the X-Y stage comprises a vacuum chuck provided with a thin porous mounting surface. The porous mounting surface in various embodiments comprises a thin paper, plastic, ceramic or metal disk having dense micro pores through which a negative pressure can be applied to a wafer placed in direct contact with the mounting surface. Embodiments of the porous member comprise one or more of porous paper, gas filters, sintered ceramic disks or plates, and sintered metal disks and plates made of various compositions.
Also in accordance with embodiments of the present invention, the mounting surface comprises a removable member. The use of a removable member further permits the ready replacement of the mounting surface when required due to wear or contamination.
The laser energy utilized in some embodiments to cut kerfs in the wafer should have a wavelength highly absorbed in the material of the substrate. Further, the wavelength should be selected so that it is absorbed to a much greater degree in the substrate than in the porous member, so that when the substrate is cut through and the laser impacts the porous member, minimal damage is caused to the porous member. For copper and similar metal substrates, the wavelength is preferably below about 560 nanometers, and more preferably between about 150 in 560 nanometers. In addition, energy density, spot size, and pulse duration are established at levels sufficient to cut kerfs completely through the wafer. Control of the system, such as by moving the stage while maintaining a stationary beam path for the pulses, causes the pulses to contact the conductive substrate in a cutting pattern at a rate of motion causing overlap of successive pulses sufficient to cut through the conductive substrate and other portions of the wafer.
Embodiments of the present invention utilize laser pulses having an energy density between about 10 and 100 joules per square centimeter, a pulse duration between about 10 and 30 nanoseconds, and a spot size between about 5 and 25 microns. The repetition rate for the pulses is greater than 5 kHz, and preferably ranges from about 10 kHz to 50 kHz or higher. The stage is moved at a rate of motion causing overlap of the pulses in the amount of 50 to 99 percent. By controlling the pulse rate, the rate of motion of the stage, and the energy density, the depth of the cut can be precisely controlled, to provide for cutting through the wafer while minimizing the amount of laser energy reaching the mounting surface securing the wafer.
In embodiments of the present invention, the solid-state laser comprises a diode pumped, Q-switched, Nd:YVO4 laser, including harmonic frequency generators such as nonlinear crystals like LBO, so that output of the laser is provided at one of the second, third, fourth and fifth harmonic frequencies of the 1064 nanometer line produced by the neodymium doped, solid-state laser. In particular systems, the third harmonic frequency of about 355 nanometers is provided. In other embodiments, the solid-state laser comprises a Q-switched, Nd:YAG laser, operating to provide one of the harmonic frequencies as output.
In embodiments of the invention, the method includes detecting edges of the conductive substrate while directing pulses at the substrate in the cutting pattern. In response to detected edges, the system prevents the pulses of radiation from being directed beyond the substrate.
Embodiments of the present invention direct the pulses of laser radiation at the backside of the wafer substrate.
Thus, embodiments of the invention include mounting the wafer on the stage, moving the wafer under conditions causing cutting of the conductive substrate in a cutting pattern on the backside of the substrate, and detecting edges of the substrate during the cutting process to prevent the pulses of laser radiation from impacting the mounting surface.
The die defined by a cutting pattern are separated from the wafer by the laser energy, while the suction applied through the pores of the mounting surface continues to secure them substantially in the same location they occupied on the mounting surface prior to the cutting. In one embodiment, an adhesive tape is placed onto the separated die after dicing the wafer is completed, in order to permit removal of the die as a set, and facilitate their handling for subsequent manufacturing steps. Furthermore, the die separated from the wafer remain adhered to the adhesive tape until removed using a pick and place robot, or other technology.
Certain embodiments of the invention further provide for controlling polarization of the laser pulses with respect to direction of the kerfs in the cutting pattern. The polarization is controlled so that kerfs are more uniform for kerfs cut parallel to different axes. Uniformity can be improved by random or circular polarization of the pulses in some embodiments. More preferably, polarization of the pulses is controlled so that the polarization is linear and parallel to the kerfs being cut. Embodiments of the invention provide for control of the polarization using a laser with an adjustable polarizer, such as a half wave plate, in the optical path.
The invention also provides a system for cutting wafers having a conductive substrate which comprises a solid-state laser, as described above, a stage having a vacuum chuck with a porous surface adapted to support and move a conductive substrate, optics directing pulses to impact of conductive substrate mounted on the stage, an edge detection system which detects edges of substrate mounted on the stage during movement of the stage, and a control system. The control system in embodiments of the invention comprises a computer system coupled to the solid-state laser, the stage, and the edge detection system. The computer is responsive to the edge detection system and parameters set by users to cause the pulses to impact of the conductive substrate in a cutting pattern at a rate of motion causing overlap of successive pulses sufficient to cut kerfs in the conductive substrate. Embodiments of the invention also include a debris exhaust system coupled with the stage.
Embodiments of the invention include a user interface with logic to set up the cutting pattern, and the operational parameters including pulse repetition rate, stage velocity and energy levels to establish kerf depth, cutting speed and other characteristics of the process.
Other aspects and advantages of the present invention can be seen on review of the drawings, the detailed description, and the claims which follow.